I cannot see why you need to upgrade this system at all, based on the parameters you set.
Unfortunately, this is not nearly descriptive enough. What you would believe is "casual" others may seem as "heavy". You might detail "casual" to only refer to the amount of time you spend you spend gaming, while in point of fact you might actually desire to play CPU/GPU intensive games... just not that often. Then again, you might mean that you only wish to play games that came out in 2007-2009 that never had heavy demands on your CPU/GPU. We don't know, because you supplied no information on this.
What you need to do is get specific with some of the games you intend to play, then look up the minimum and recommended specifications of those games, in order for you to know if your current system is enough to play them. We can't really assume anything when it comes to gaming.
From what you detail, you already have far more than enough to handle these tasks... depending of course on what you actually mean. Again, the devil is in the details. There is a vast difference between editing HD video and non-HD video. There is a vast difference between editing pictures from your phone, and editing magazine quality publications. Assuming that you mean that you want to process photos for sending to Facebook and other social media sites, as well as make videos to send up to Youtube... I can't see any reason to upgrade your hardware. You've already got far more system than what most people would purchase in a laptop for exactly the same tasks.
what should I buy
As was pointed out in the comments, we can't make shopping recommendations. The reasons are explained in the FAQ.
The fact that the system is just "collecting dust" raises the question of why it is sitting idle. If you find the system can't adequately play the games you want to play, then you probably cannot continue to say that you are looking for a "casual gaming" system. If you find that the system does not pack enough muscle for the video and photo editing that you do now, then you are not looking for something relatively simple and you need something more at the level of a professional rig. You would be looking for cutting edge, top-of-the-line hardware... and for that, there are multiple web sites out there. You can start at one of any online retailers (Newegg, etc) or even retailer portals (Pricewatch, etc) and start looking at the more expensive stuff that is offered. Check the specifications, compare prices, and decide on what you want.
Again, as you have already heard from Duncan though... what you've got is plenty for what you say you want to do.